1. Technical Field
The present invention relates to a cleaning system, a fixing device, and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, a cleaning system for use in such a fixing device, and an electrophotographic image forming apparatus which employs a fixing device with a cleaning capability.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium.
Various types of fixing processes are known in the art, among which a pressure-assisted thermal fixing process is widely accepted. This type of fixing device employs a pair of generally cylindrical members, such as a looped belt and a roller, one having a heat source such as a halogen heater or the like for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One problem associated with the pressure-assisted thermal fixing process is undesired transfer or offset of toner particles to a fuser member. Ideally, a toner image after fixing permanently adheres to a recording medium on which it is printed. However, toner offset often takes place, for example, due to improper heating at the fixing nip, where adhesion between the fuser member and the fused toner exceeds that between the recording medium and the fused toner, causing a small portion of toner to transfer from the recording medium to the fuser member.
Two types of toner offset are known: cold offset and hot offset. Cold offset occurs where insufficient heating at the fixing nip causes the toner image to fuse only superficially, leaving an inner portion of the toner layer in a loose, unfused state, which can partially crush up and eventually migrate to the fuser member. Such toner migration is typically accompanied by concomitant image defects in which the toner image, which is not completely fused or fixed, easily rubs off the printed surface being output. Hot offset, on the other hand, occurs where excessive heating at the fixing nip affects viscoelasticity of the toner image being fused, so that the toner exhibits a high adhesion to the fuser member surpassing a cohesive force of toner particles, resulting in partial migration of toner to the fuser member.
The problem described above, in particular, cold offset, is pronounced where printing is performed using specific, newly developed types of toner, including those formulated with extremely small particle sizes, or those with spherically shaped particles typically produced through polymerization, which are increasingly employed in modern electrophotographic printers to meet ever-increasing demands for high-quality imaging processes. Compared to those with varying sizes and aspherical shapes, the small-sized, spherically-shaped toner is susceptible to causing cold offset since it does not easily conduct heat, and therefore is difficult to fuse and melt, particularly when used to print on a rough, irregular surface of non-coated paper.
Not surprisingly, toner offset detracts from image quality due not only to a lack of toner migrating from the recording medium, but also to soiling of the resulting print with offset toner which, once transferred from a recording medium onto the fixing member, is again transferred to another recording medium that enters the fixing nip subsequent to the foregoing recording medium.
Various cleaning techniques have been proposed to keep the fuser member clean of toner particles and other contaminants, which employ a cleaning web, such as an elongated strip of unwoven fabric, to wipe the surface of the fuser member. In a typical configuration, the cleaning web is drawn from a replaceable supply roller and pulled by and wound on a takeup roller, with a tension roller elastically biased against the fuser member to form a cleaning nip therebetween, through which the web is passed to press against the fuser member.
For example, one such technique provides a web cleaning system that continuously cleans a fuser member with a cleaning web during operation of a fixing device. According to this method, the cleaning web is taken up by a takeup roller upon completion of each print job to constantly supply a new, unused sufficiently large area of the web to the cleaning nip. Such constant supply of new cleaning web prevents formation of a gap between the fuser member and the web, which, if created, would permit small spherical toner particles to escape from being wiped off at the cleaning nip.
Although capable of effectively cleaning the fuser member, this cleaning method results in wasteful use of the cleaning web, which is detrimental to environment. Also, accelerated consumption of the cleaning web requires frequent service for the cleaning system and thus eventually increases maintenance cost of the image forming apparatus.
Another technique proposes a control method for a web cleaning system which controls supply of a cleaning web to the cleaning nip. According to this method, the controller adjusts an amount by which the cleaning web is taken up depending on image density (i.e., a ratio of a toner-covered area to an entire image area) of a specific print job processed through the fixing nip. Although designed to prevent an unnecessary, superfluous supply of cleaning web upon processing of relatively light or low density images, however, such control does not work properly because the image density is not always proportional to the amount of toner offset to the fuser member.
Still another technique proposes a web cleaning system employing a cleaning web directed to a pressure member opposite a fuser member, which indirectly cleans the fuser member as the cleaning web wipes the pressure member which collects toner retransferred from the fuser member. According this method, providing the cleaning web to the pressure member, instead of the fuser member, prevents the cleaning web from damaging the surface of the fuser member facing the printed surface of a recording medium image, which would otherwise cause imaging defects, such as vertical straight lines appearing on the printed page.